I've heard a guy saying "подруга" referring to all girls in his circle, he had a lot of female colleagues (he was not in relationship with any of them), he just couldn't find another word. But still it sounded a bit strange (may be it was OK in his circle of liberal city boys and city girls), because in most cases when a guy says "подруга", in most contexts that would be his girlfriend. So an innocent word would be "моя знакомая" - can't imply any relationship. It seems to me this area of language in Russian is rather complicated, sometimes even native speakers are somewhat in doubt as to what word to use.